Heretofore, in the semiconductor industry, a photolithography method employing visible light or ultraviolet light has been used as a technique to transfer a fine pattern required to form an integrated circuit of a fine pattern on e.g. a Si substrate. However, the conventional photolithography method has come close to its limit, while miniaturization of semiconductor devices has been accelerated. In the case of the photolithography method, the resolution limit of a pattern is about ½ of the exposure wavelength. Even if an immersion method is employed, the resolution limit is said to be about ¼ of the exposure wavelength, and even if an immersion method of ArF laser (wavelength: 193 nm) is employed, about 45 nm is presumed to be the limit. From this point of view, EUV lithography, which is an exposure technique employing EUV light having a wavelength further shorter than ArF laser, is expected to be prospective as a next generation exposure technique for 45 nm or below. In this specification, EUV light means a light ray having a wavelength within a soft X-ray region or within a vacuum ultraviolet region, specifically a light ray having a wavelength of from about 10 to 20 nm, particularly about 13.5 nm±0.3 nm.
EUV light is likely to be absorbed by all kinds of substances, and the refractive indices of substances at such a wavelength are close to 1, whereby it is not possible to use a conventional dioptric system like photolithography employing visible light or ultraviolet light. For this reason, for EUV lithography, a catoptric system, i.e. a reflective photomask (hereinafter also referred to as “EUV mask”) and a mirror, is employed.
A mask blank is a stacked structure for a photomask, which has not been patterned yet. In the case of an EUV mask blank, it has a structure wherein a reflective layer for reflecting EUV light and an absorber layer for absorbing EUV light, are formed in this order on a substrate made of e.g. glass (Patent Document 1). As the absorber layer, a material having a high extinction coefficient for EUV light, specifically, for example, a material having Ta as the main component, is used.
Patent Document 1 discloses that a tantalum-boron alloy nitride (TaBN), a tantalum-boron alloy oxide (TaBO) and a tantalum-boron alloy oxynitride (TaBNO) are preferably used as a material of the absorber layer since they have a high extinction coefficient for EUV light and a low reflectivity for deep ultraviolet light at a wavelength region (190 nm to 260 nm) of pattern inspection light.
Recently, in an EUV mask blank, the thickness of the absorber layer has been desired to be reduced. In EUV lithography, an EUV mask is irradiated with exposure light not from a perpendicular direction to the EUV mask but from a direction at an angle of a few degrees, usually 6 degrees, to the perpendicular direction. If the thickness of the absorber layer is thick, at the time of EUV lithography, a shadow by the exposure light arises on a mask pattern formed by removing a part of the absorber layer by etching, and the form accuracy or the dimension accuracy of a mask pattern (hereinafter referred to as “transfer pattern”) to be transferred to a resist on a substrate such as a Si wafer by using the EUV mask, tends to be deteriorated. Since this problem becomes more significant as the line width of the mask pattern formed on the EUV mask becomes smaller, the thickness of the absorber layer of the EUV mask blank is required to be more reduced.
Ideally, a material having a high extinction coefficient for EUV light should be employed for the absorber layer of the EUV mask blank, and the thickness should be such that EUV light applied to the surface of the absorber layer is completely absorbed. However, as described above, since the thickness of the absorber layer is required to be reduced, the EUV light applied to the layer is not completely absorbed by the absorber layer, and a part of the EUV light becomes reflection light.
What is required when a transfer pattern is formed on a resist on a substrate by EUV lithography is the contrast of reflection light in the EUV mask, that is, the contrast between reflection light from a portion of the mask wherein the absorber layer is removed at the time of forming the mask pattern so that the reflective layer is exposed to the outside, and reflection light from a portion of the mask wherein the absorber layer is not removed at the time of forming the mask pattern. Accordingly, it has been considered that so long as a sufficient contrast of reflection light is secured, there is no problem even if the applied EUV light is not completely absorbed by the absorber layer.
Based on the above concept, an EUV mask using the principle of phase shift to reduce the thickness of the absorber layer is proposed (Patent Documents 2 and 3). Such an EUV mask is characterized in that the reflectivity for reflection light from the portion wherein the absorber layer is not removed at the time of forming a mask pattern is from 3 to 15% and that the phase difference between the reflection light from the portion wherein the absorber layer is not removed at the time of forming a mask pattern and the reflection light from the portion wherein the absorber layer is removed at the time of forming the mask pattern so that the reflective layer is exposed to the outside is from 175 to 185 degrees. It is disclosed that in such an EUV mask, the principle of phase shift is applied to the reflection light from the absorber layer, whereby it is possible to sufficiently maintain the contrast with the reflection light from the reflective layer and that it is thereby possible to reduce the thickness of the absorber layer.